This invention relates to light spot scanning, and more particularly to scanning with a spot of light generated holographically.
The primary function of a scanning system is the controlled sampling, or restoration, of information. In an optical scanning system, the information is processed either in parallel by a light beam which can simultaneously illuminate many data sites, or sequentially by a beam which, due to its size, illuminates only a single data site at a time. Interest in sequential optical scanning has expanded in recent years, primarily because of new capabilities provided by laser light. Laser scanners are capable of generating high resolution images at high scan rates. Most of the scanning systems devised to manipulate a laser beam include a galvanometer, rotating mirror, acousto-optic elements, or electro-optic elements as the light deflector. It was first demonstrated in 1967 that a rotating hologram can also serve as a deflector element in an image scanning system.
Laser line scanners used for imaging applications are generally required to generate a repetitive single scan line. A problem which has been encountered with multi-faceted rotating mirror line scanners is that due to the facet-to-facet nonuniformities and spinner wobble, noncollinear multiple scan lines are formed. An obvious solution to this problem is to fabricate the spinner assembly to such precise mechanical and optical tolerances that the residual error does not detract from the desired level of image quality. The expense of this approach, however, is a decided disadvantage. Holographic scanning provides an alternative by which this problem can be minimized.
In a typical arrangement for making a flat holographic spinner, a point light source serves as the object and a normally incident plane light wave as the reference beam. When a hologram thus constructed is illuminated with a plane light wave which is the conjugate of the original reference beam, the hologram functions to reconstruct a wavefront which converges with or without the use of additional optical elements to form an image of the original point object light source. When the holographic spinner is then rotated about the axis of the reference beam, the reconstructed image spot scans a circle in space. This spot, however, is subject to perturbations created by mechanical wobble of the spinner.
In co-pending application Ser. No. 708,245 filed on July 23, 1976 and assigned to the same assignee as the present application, a specific system geometry was disclosed whereby, if the angles of incidence and diffraction of the principal light ray are made substantially equal, the reconstructed image wavefront propagating from the holographic spinner is substantially insensitive to spinner wobble. This geometry, however, places a constraint upon the system since it may not be possible, or desirable, in all cases, to make the angles of incidence and diffraction equal. Also, the grating used was limited to transmissive types.
It is, therefore, the one object of this invention to provide a holographic scanner which is insensitive to mechanical wobble during rotation.
Another object is to provide a holographic scanner which is invariant with respect to mechanical wobble without any constraints placed on the angles of incidence and diffraction of the principal light rays.
Still another object is to provide a holographic scanner which can be of the transmissive or reflective type and which is invariant with respect to mechanical wobble.